Recently, tools for administrators of email systems have become available to prevent their clients from receiving unsolicited/unauthorized inbound email (spam). For example, several types of firewalls and customizable filters are available to prevent inbound spam email. Adaptive filters, such as those based on bayesian probability models, have also been relatively effective in preventing some of the more sophisticated spam email, such as spam that tries to obfuscate source addresses and/or general content. Although tools to manage inbound spam email have improved in efficiency and efficacy, the “spammers” have become equally ingenious in devising ways to beat the latest preventive methods. Also, the amount of system resources that must be dedicated to handling the seemingly ever increasing amount of inbound spam email can seriously reduce the amount of resources available for legitimate email traffic.
Although email administrators have had access to a wide variety of methods to prevent inbound spam emails for some time, preventing the abuse of an email system employed to originate outbound spam email has not received the same degree of attention. Notably, most spam prevention methods are directed to stopping inbound spam email, not outbound. Also, it has become known that a significant portion of spam email originates in email systems that enable anyone to anonymously sign up for multiple email accounts. Unfortunately, spammers are known to abuse this anonymity by automating the sending of large amounts of spam email from multiple email accounts, often to users of the same email system. To prevent such abuse, email administrators could benefit from a method that could challenge a particular client's outbound email usage that exceeds certain limits in a manner that is incompatible with an automated response, i.e., a challenge that is easily solved by a human being and difficult to answer for an automated computer program such as those employed by spammers.